The stag beetle Lucanus cervus (LINNAEUS, 1758)
Text © Klaas Reißmann, 2007
With its bodylength of up to 80 mm, the distinctive stag beetle is the largest species of the German beetle fauna.
Most people know the beetle, a few may even have found one themselves. Due to the large, eye-catching mandibles of the male, the beetle's vernacular
name has been derived from the stag, e.g. as "stag beetle" in English, or "Hirschkäfer" in German. Nowadays not too many people have ever seen
the stag beetle in the wild. In the Red List of the German beetle fauna (1998) the beetle is classified as "2" (endangered), due to the fact, that
larger populations today are rather rare and limited to small regions. Currently there is no evidence for a positive development of the German stag
Until the thirties of the last century, the stag beetle was, at least locally, a common species.
Although first losses in the populations were registered at that time already, our grandparents usually
knew the beetle not only from stories, but from their own experience. So how could it happen, that the formerly
common species in our forests is now more or less facing extinction?
2. Life cycle
In Central Europe, the imagines (adults) of the stag beetle appear in early summer, usually in June and July. Depending on the
weather conditions, the first imagines may also appear already in May, especially in southern Germany, whereas "latecomers" and
the more long-living females may be found until August. At higher elevation, the activity period is delayed accordingly, the imagines are
found from July to August, sometimes even until September. The males usually hatch about one week earlier than the females, because their sealed
cell is less deep located in the ground and grows warm earlier in spring.
In the proximity of such spots courtship behavior takes place, and - in case two or more males encounter each other - the males fight each other.
In the fight, the males try to lever out the opponent with their mandibles and to throw each other off the tree. In such cases, the clicking noise of the elytrae
can be heard from a few meters distance. Occasionally the opponents can even cause deadly injuries in the abdomen of their rival. The mating
usually takes place directly on the spot, where sap is oozing from the bark, with the male adopting its typical protecting position over the female.
Typical predators of stag beetles are various bird species, like woodpeckers (Picidae), jaybirds (Garrulus glandarius) and owls (Strigidae), but
also kestrels (Falco tinnunculus) and crows (Corvus) and other species. Crows and jaybirds have been observed to separate head, pronotum and elytrae
before swallowing the abdomen. The remains can be often found during the activity period of the beetles. Wild hogs (Sus scrofa) and badgers (Meles
meles) prey the pupae in the ground. Rumpled ground around rootstocks of oaks is a clear sign of wild hog activity in the area.
Other predators like hedgehog (Erinaceus europaeus), fox (Vulpes vulpes), shrews and moles (Talpa europaea) feed on the beetles as well.
Hence the mortality of the beetles is high. TOCHTERMANN found out, that only 100 females per square kilometer out of 150 achieve
oviposition with 12 to 14 eggs, in total 1,200 to 1,400 eggs. Out of these only 600 larvae reach the imaginal stage. With a gender ratio of 1:4 (females:males)
this means, that the number of females esentially remains constant, of course subject to seasonal fluctuation. After mating the females start
their strenous search for suitable breeding substrates. Often they seek the same rootstock where they developed. Breeding substrate more than 2,000 m
away from the mating spot is not frequented.
When the female finds suitable breeding substrate, it digs itself approximately 30 to 50 cm, at most 100 cm into the soil. It deposits 12 to 14 eggs
close to the roots of the rootstock. Afterwards, the female frequents again mating spots. The imgaginal life of the female therefore
can be regarded as a cycle of feeding, mating, searching for breeding substrate and oviposition. About 70% of their lifetime is spent for
the search of suitable breeding substrate. About 50% of the females achieve a 2nd oviposition, only 5% a 3rd one. In captivity also a 4th
oviposition has been observed. The maximal number of eggs produced by one female has been 28. Formerly it was believed, that a female can
produce 50 to 100 eggs, which seems doubtful for sheer anatomic reasons. However, breeding observations with species from other
families, like some jewel beetles (Buprestidae), indicate, that the food intake of the female provides the basis for a sustained production of eggs
(ZÁBRANSKÝ, P., personal correspondence, 2007). Often oviposition is not achieved at all, for example if the female cannot find
suitable breeding substrate. In this case they dig themselves into the ground, come out without having achieved anything, continue their
search, dig themselves in and so on, until they die from exhaustion, and the eggs in their abdomen perish.
3. Population losses and causes
Up to this point the cause for the massive losses in the populations remains unclear. However, recent research brought up an unexpected explanation:
There is a connection between the requirements of the larvae regarding its substrate, and the cellular components of oak wood.
Oak wood contains up to 18 different tannic acids. During the growth period the tannic acids are equally distributed among to whole tree.
However, in autumn the tannic acids are concentrated in the roots of the oak. As trees are usually cut in late autumn or winter in Germany,
the rootstock remaining after cutting contains high amounts of tannic acid, hindering fungal activity. Although the fungi are still present on the
wood, the wood is only digested up to 0.5 cm below the surface. Hence the generated substrate suitable larva is much too small, and the stag beetle
can not develop in such rootstocks.
4. Support programmes
So the question is, what can be done to preserve the species and to make sure, that to populations recover and the populated areas increase.
Stag beetle loggeries can be regarded as an effective way to support the species. It is important, that these loggeries comply with the
requirements of the stag beetle.
It should be noted, that building such a stag beetle loggery only makes sense, if a suitable forest is close by. Oak forests covering an area of at least
5 ha with trees with 150 to 250 years in age are especially suitable. Also stag beetles should be present in the area close by already. Although
stag beetles can cover larger distances in flight especially on windy days, usually the loggery should be within a range of 2,000 m of an existing
stag beetle population. It is also important to have oak trees with spots where sap is oozing from the bark within the same range
around the loggery.
In extremely rare cases saggital semi-hermaphrodites can be found in stag beetle populations. In these specimen, one of the mandibles is build
like in males whereas the other one is small, similar to those found in females. Normally the semi-hermaphrodites are male on on the left side and
female on the right side, the opposite case is extremely rare. However, the specimen is often not a real hermaphrodite, as the genitalia can be clearly
assigned to one gender.
Literature (in German only):